The invention relates to a method and an arrangement for inspection of a test object for conspicuous stains or pits and measurement of any stains or pits. The method and arrangement are especially suitable for inspection and measurement in a radiated environment. The method and arrangement are especially suitable for stains or pits as fretting marks on a nuclear fuel rod.
A fuel assembly for a boiling water reactor (BWR) comprises an elongated tubular container, often with rectangular or square cross section, which is open at both ends forming a continuous flow passageway. A coolant, for example water, is arranged to flow through the container. The fuel assembly comprises a large number of elongated tubular fuel rods, arranged in parallel in a certain, normally symmetrical pattern. Each of the fuel rods comprises a long tubular outer cover, named cladding, which is filled with nuclear fuel, for example in the form of pellets. The fuel rods are normally arranged vertically and retained at the top by a top tie plate and at the bottom by a bottom tie plate. Between the top tie plate and the top of the fuel rods is some play, to compensate for changes of length due to temperature changes under operation. To allow the coolant to flow freely past the fuel rods, the fuel rods are spaced from each other and prevented from bending or vibrating when the reactor is in operation by means of a plurality of spacers. The spacers are arranged at several levels in the fuel assembly, between the top and bottom plates.
A fuel assembly for a pressurised water reactor (PWR) is designed in substantially the same way as the fuel assembly for a BWR, apart from the fact that the fuel rods are not enclosed by any tubular container and the number of fuel rods is larger.
A typical spacer comprises two grids arranged in parallel and spaced apart from each other and surrounded by a common rim. Resilient material strips are arranged between the two grids forming cells. The fuel rod is guided by the strips and the cells in the grids. A plurality of supporting embossments is arranged at the grids and the strips. The embossments are normally arranged in contact with the fuel rod cladding in order to position the fuel rod.
Areas at which the spacers are in contact with the fuel rod cladding are the most likely areas for wear, as for example corrosion or abrasion, at a fuel rod. The wear often appears first in a change of colour as a conspicuous stain. When the wear of the material increases, the stains become pitted. In extreme cases pits may become so deep that the cladding becomes porous and fissile material leaks into the coolant, which should be avoided. Pits or stains on nuclear fuel rods are often referred to as fretting marks.
It is of interest to know if and in such case how the fuel rods are affected with wear or corrosion, after some time of operation in the nuclear reactor, for example to avoid leaking fuel rods. This is specially of interest when a new kind of fuel rods is brought into service, or a known sort of fuel rods is used under changed operating conditions. Fuel rods which possibly failed during operation should also be examined. If such a fuel rod is inspected, inspection is made visually, for example by means of a camera. Visual inspection is time-consuming and not very accurate. During visual inspection the width of the fretting marks may be measured. If the depth of a fretting mark is to be measured this is done manually with mechanical methods. The manual measurement incorporates further, time-consuming, handling of the fuel rod.
The invention relates to a method and an arrangement for inspection of a test object for conspicuous stains or pits and measurement of any stains or pits. The method and arrangement are especially suitable for inspection and measurement in a radiated environment. The test object may be a fuel rod where stains and pits on its cladding are referred to as fretting marks. The method includes measuring of at least one dimension, such as width or depth, of a stain or pit.
The inventive method is defined in claim 1 and the inventive arrangement in claim 5.
The arrangement comprises an inspection fixture comprising a monitoring device, a scanning device and a control urut, which may be arranged at a distance from the fixture. The test object is guided in the inspection fixture and is monitored by means of the monitoring device. Signals from the monitoring device are transmitted to the control unit and shown on a monitor. Any stain or pit on the test object, shown by the monitoring device, is subsequently scanned by means of the scanning device. The scanning device may be for example an ultrasonic transducer or a laser scanner. Scanning results are transmitted to the control unit. The depth of any stain or pit relative to test object surface is subsequently calculated by means of evaluation routines implemented in the control unit. The width of any stain or pit can be calculated in a similar manner.
An advantage of the present invention is that steps of monitoring the test object by means of a monitoring device, as for example a camera, and measuring of any stain or pit are combined in one arrangement, such that the test object needs only to be handled one time under the inspection. The measuring of the depth of stains or pits is incorporated in the method.
The inspection of the test object is partly automated and is remote controlled and supervised via the control unit. The inspection of the test object, for example a radioactive fuel rod, may be made under water in, for example, a spent fuel pool. Measurement of the depth of any stain or pit is remote controlled via a control unit. By means of, for example, a readily available ultrasonic transducer or laser scanner, an accuracy of 0.05 mm or better is achieved.
Due to the partly automated and remote controlled process, is it possible to inspect a greater number of fuel rods than before in a modest time. This is an advantage, for example, where new cladding materials are tested or in cases of operation distortions due to problems with fuel rods.